Shallow burial diagenesis of skeletal carbonates: selective loss of aragonite shell material (Miocene to Recent, Queensland Plateau and Queensland Trough, NE Australia) - implications for shallow cool-water carbonates

In burial environments, carbonate sediments undergo mineralogical stabiliza tion and increasing lithification with depth. As yet, however, little knowl edge exists with respect to the corresponding effects on fossil preservatio n and taphonomic modification of the original sediment composition. Countin gs of particles (> 63 mum) in Miocene to Recent periplatform sediments (ODP Leg 133, NE Australia) exhibit a clear trend of reduction of skeletal arag onite downcore. Low- and high-Mg-calcite grains occur in a continuous order of magnitude over the studied interval (< 600 m sub-bottom depth). Origina l microtextures are retained in high-Mg-calcite biota, although converted t o low-Mg-calcite. Thus, the conversion to low-Mg-calcite appears to occur w ithout introducing a significant quantitative bias. Aragonite skeletons (pe lagic gastropods), however, exhibit a successive exposure of deeper crystal layers and a chalky preservation with burial depth, which we interpret to result from dissolution. Hints fur the originally more numerous existence o f aragonite biota exist in soft sediments and chalks by the presence of int ernal moulds, shells replaced by microspar, and mouldic porosity in early c emented hardgrounds. In deep sections barren of aragonite, the number of ca sts and replaced shells remains unchanged and is insignificant as compared to aragonitic biota probably originally present within the sediment (<2% in oozelchalk vs. 30-50% of grains in modem periplatform sediments). Therefor e, palaeontological information must be significantly biased through select ive removal of aragonite. Rates of preservation and destruction depend on e xternal. factors during sediment accretion (sedimentation rates, clay conte nt, total organic carbon content) and rates of fluid flow within the sedime nt. These observations are relevant with respect to the diagenetic potentia l and patterns of fossil preservation in little cemented calcitic cool-wate r carbonates, because they may originally have contained more aragonite bio ta as important constituents of an ecosystem than is commonly suspected, an d calcite/aragonite ratios in ancient carbonate sediments may not necessari ly reflect original input signals (climate or sea level). (C) 2000 Elsevier Science B.V. All rights reserved.